CDIS 4027 Inner Ear


So in the previous lecture, we talked about the middle ear’s anatomy and physiology, and we talked about the assessment on the middle ear using tympanometry and acoustic reflexes. So now we will continue with anatomy and physiology with the inner ear now. So the inner ear begins with the medial wall of the middle ear, to which the stapes is attached to, to the oval window. The inner ear is where the end organs of the auditory system and the vestibular system are located. Just like other special organs, such as the eye and the tongue, the purpose of the auditory system is to actually convert an external, physical stimulus, in this case acoustical energy, into a form that the nervous system can understand, so it can be conducted through the nerve, in this case the auditory nerve, to higher structures where the acoustical signal can be interpreted as a sound. And the language that the nervous system understands in electrochemical energy. So, the ultimate purpose of the inner ear is to convert acoustical energy Into electrochemical energy that can be carried by the nerves to the structure in the brain, temporal lobe, where the sound is interpreted. This process is what we call as auditory transduction. Auditory trasnduction refers to the transformation of energy from one form to the other. So here we are converting acoustical energy into electrochemical energy. And it’s much like, ironically, the common analogy to describe the inner ear is the microphones, which does the same purpose; they convert the acoustical sound energy into electrical energy that is carried by the cables to an amplifier and then it could be amplified by a speaker, but why I say ironical is because microphones are actually based on how the inner ear or the auditory system works, but even with all the technological advances, we have still yet to construct a microphone that is as small, versatile, or sophisticated as our inner ear. The inner ear, even in adults, is but the size of your pinky nail finger, so we are talking about a really small structure, which is highly sophisticated in terms of its architecture and its mechanics, something that I hope to explain in this few lectures. The inner ear is also known as a labyrinth because it’s similar to a winding cave. Functionally, it’s easier to study the inner ear anatomy as two parts: the bony labyrinth, or the ossessous labyrinth, and the membranous labyrinth. Although anatomically, the membranous labyrinth is located within the bony labyrinth, so the bony labyrinth is but a series of cavities, kind of engraved within a bone and, in fact, this bone is one of the hardest bones in our skull, namely the temporal bone, specifically, it’s a petrous part of the temporal bone, and the membranous labyrinth, as the name goes, it’s a membranous sac that kind of floats within this bony labyrinth. So here in this illustration, you can see both of the inner ears kind of located in the bottom half of the skull. Just to orient yourself, this is the foramen magnum, it’s the main opening of the skull through which the brainstem, cerebral vertebrae, enter and connects to the brain. Here is an illustration of how small this whole inner ear is; it is but a 2 cm cube, and what’s fascinating is is when you’re looking at the embryological development of the inner ear, the inner ear is kind of adult sized by the beginning of the second trimester, so for all purposes, the inner ear is fully grown by the 4th month of gestation. So it is true that the young embryo can actually hear sounds, but the sounds that they hear are probably muffled because they are travelling through all the liquid and soft tissue around the womb. In this illustration, you are seeing the membranous labyrinth taking the curves of the bony labyrinth that it lies within the cavity. Here is an illustration of actually skull segments, and you can see, actually, that the inner ear is located as kind of a series of cavities within the skull, within the temporal bone, and you can see the different shapes over there. So it’s not like, for instance the eye, where you can actually remove the whole eye. Here, we are talking about structures that are located deep within the bone, and that explains why it took us a long time, even understanding the normal physiology of the inner ear, because even accessing the inner ear required this kind of breaking into the skull, and that, by itself, results in an abnormal or it affects the normal functioning of the inner ear. So we have to wait for technological advances to understand what’s going on within the inner ear, and that also explains why, in some of our earlier theories, about hearing actually were faulty or underestimating the function of the inner ear. Here’s another illustration of kind of a dried out part of a temporal bone. Here you’re seeing those cavities, these are actually the scala of the cochlea and we’re going to be talking about that in just a bit; how there’s actually three compartments within the cochlea, arranged in a spiral fashion. Just to orient yourself again, this is the stapes of the middle ear cavity, and here’s you’re actually seeing the porous mastoid bone and the mastoid hair cells over there. So the bony labyrinth consists of three parts: We have the central, kind of dome shaped, vestibule, much like the vestibule of our home and on either side, we have, on one side we have the cochlea, the spiral cochlea, and on the other side, we have the set of 3 semi-circular canals. The bony labyrinth has a fluid known as a perilymph, and it’s in this perilymph that this membranous labyrinth actually kind of floats. One end of the semi-circular canal has a swelling, and it’s that swelling that we kind of call as an ampulla over here. Ok, here, so vestibule is an oval cavityl that is located between the cochlea and the semi-circular canals. And it’s this vestibule that we have those two windows, namely the oval window and the round window. Again, to recollect, the oval window is where the foot of the stapes of the ossicles is attached to. And the purpose of the round window is actually is to dissipate pressure when the stapes actually moves in and out as sound enters the inner ear. Again, at one end we have got the 3 semi-circular canals, and the easy way to remember them is S.P.L.: superior, posterior, lateral. And they are roughly at, oriented at, 90 degrees to each other, and that serves a purpose that I’ll get to in just a few minutes. It’s in this vestibule and the semi-circular canals where the vestibular apparatus is located, which is important for balance perception. Again, in this juncture, I just wanted to remind that the vestibular assessment actually falls within the domain of audiologists. A number of these audiologists are responsible for these vestibular assessments and diagnosing vestibular conditions. In fact, here, in our program at ETSU, we have 2 vestibular specialists that are kind of world-renowned for their research in vestibular assessments. Both of them, Dr. Akin and Dr. Murnane, are affiliated with the VA, and that’s an excellent opportunity that our students gain by being enrolled in those vestibular assessments. Each semicircular canal, as I said, has one swelling at its end, which is known as an ampulla. And this is a significant anatomical point because that’s where the vestibular end organs of the semicircular canals are located. The bony cochlea, commonly referred to like a snail-shaped shell, it’s also like the top of a frosty ice cream. Earlier, it was believe that this coiling was to save space and also coiling helps to tuck in the sensory, sensitive, organs and save them from damage. But now actually, recent research has shown, that it probably has another important function that we will touch in when we talk about the physiology. It forms the anterior part of the labyrinth, and it’s almost horizontal. Horizontally, it sits on the vestibule. It’s not a completely concrete bone; it’s actually got holes at the bottom of the cochlea through which the 8th, the auditory-vestibular, nerves enters, and kinds of reaches to the hair cells. So here, you’re seeing the bony cochlea in isolation. So you’ve got the modiolus, which is that center core, and again, the center core is where the perforations are there through which the 8th/auditory nerve enters and kind of spreads out to the different turns of the cochlea. In humans, the cochlea takes about 2 and 3/4 of a turn. In some mammals, the cochlea might actually have 4 or 5 turns, and that’s probably what is responsible for the extended frequency range that many of those mammals can listen to. Again, it’s about half the size of the inner ear is only 1 cm wide and it’s only 5 mm from top to bottom. And it’s easier to study the physiology of the cochlea as if it was a straightened out canal, so if we were to straighten it out, it would be about 30 mm in length. It’s such that it is broader for those basal turns and it gets kinda narrower as you go to the apical or top turns. So that was the bony labyrinth. The membranous labyrinth refers to those membranous sacs that float within the bony labyrinth, and they pretty much take the same shape as the bony labyrinth, except for the vestibule while we have one domed-shaped, bony vestibule , actually inside there are two sacs: we’ve got the saccule, which is connected to the cochlea, and we’ve got the utricle, over which the semicircular canals are set. The membranous labyrinth is filled with a different kind of fluid known as endolymph. But the membranous labyrinth, just remember it floats in another fluid called perilymph, ‘peri’ mean perimeter, outside; ‘endo’ is inside, endolymph. And the chemical composition of those two fluids are different; they plays a role in the cochlear physiology that we will be talking about. Here’s an illustration of the membranous labyrinth kind of sculpted out of the bone of an animal, so it kind of looks like a gelatinous shrimp. And here, you can see that the openings, the oval window and the round window. I’ve got a link over here for those who have the stomach for it. It’s actually a dissection of the ear, so hopefully it works. You might enjoy watching a dissection of a cat ear. So, again, the membranous labyrinth is those interconnected sacs that lie within the bony labyrinth. Here, again, you can see the two compartment, membranous vestibules. We’ve got the saccule over here, connected to the cochlea. And then you’ve got the utricle over here, where the semicircular canals sit on. And then you can see at one end of that semicircular canal is the swelling known as ampulla. If you were to dissect the cochlea right at the center, almost like that, it would seem like that, Actually the membranous labyrinth refers to that central compartment, and just because it lies within the bony labyrinth, it ends up dividing the the whole turn, each turn of the cochlea into 3 compartments. The three compartments are actually, the top one is the scala vestibuli, the middle one is the scala media, center, and the bottom one is the scala tympani. Here you’re seeing the 8th cranial nerve, the auditory nerve, entering and it’s kind of reaching out into the different turns of the cochlea. So the membranous vestibule and the semicircular canals contain the end organs of the vestibular system. And as I mentioned earlier, the membranous vestibule has the utricle and the saccule. And the utricle and the saccule have those hair cells that are actually what is responsible for capturing our head movement in any given plane, and it’s sent through the vestibular nerve and is interpreted as a change in our head orientation. You wouldn’t be talking too much about the vestibular system, apart from this brief anatomy, but just to give you an idea of what other structures are there in the inner ear. So the semicircular canals, as I said, we’ve got three of them oriented at right angles to each other, and they are there for a purpose because they’re there to capture angular acceleration, which refers to our ability to perceive head rotation in different planes. So now with three different semicircular canals oriented at right angles to each other, we can perceive head rotation on the three different planes. So whichever direction we turn, it stimulates one or more of the semicircular canals, and our brain interprets that as motion in a plane. It’s almost like hair cells floating in a fluid, and they are deflected in one direction when we are moving in one plane. Here’s a link to an animation that kind of has a little bit more information about the function of the semicircular canals. So when you’re turning your head, the hair cells, which are located within the semicircular canals, are displaced and, depending upon which plane we are moving, any given one or more of the semicircular canals are stimulated and that helps us perceive in which plane we are moving in. The hair cells within the semicircular canal are known as cupula, and, again, the cupula are responsible for capturing angular acceleration. The membranous labyrinth in the vestibule again, we talked about how that’s divided in to the utricle and the saccule. The saccule is what is connected to the cochlea; it is connected by this narrow duct known as the ductus reuniens. Both the utricle and the saccule are responsible for linear acceleration. Linear acceleration refers to our ability to perceive motion in a single plane, let’s say up and down, or when you are moving in a car, like front and back. So that’s different from angular acceleration that is captured by the semicircular canals, where that’s the revolution of your head in any given plane. So the sensory regions of the utricle and the saccule are known as the macula. While the sensory regions of the semicircular canals, as I mentioned earlier, are located within that ampulla that’s swelling at one end of the semicircular canal. They’re known as the cupula. So in this example, you are seeing that the macula are capturing linear acceleration like you moving front and back or up and down, as if in an elevator. And when you are doing vestibular assessments, actually you are trying to target either the semicircular canals or the utricle and saccule, you’re assessing how well these structures are capturing movement. So the membranous cochlea lies within this bony cochlea, kind of coiling around that central core or modiolus. It consists of three ducts or scalae: as mentioned earlier, we’ve got the scala vestibuli, the scala tympani, and between them lies the scala media. The scala media is also known as the cochlear duct, and it floats in between the scala vestibuli on the top and the scala tympani on the bottom. So the scala vestibuli is separated from the scala media by this membrane known as the Reissner’s membrane. While the scala media is separated from the scala tympani, which lies on the bottom, by the basilar membrane. And specifically, this basilar membrane is an important membrane because its over which lies the sensory hair cells of the cochlea. So here’s an illustration, I mean a scanning image, of a cross section of the cochlea, so if you were to take the cochlear turn and you were to kind of make a cross section right over there and look into it, it’s going to look like that. So here, we are seeing the 3 scalae; the scala vestibuli at the top, it’s the larger of the 3 compartments; we’ve got the scala media over here that lies between the scala vestibuli and the scala tympani on the bottom. So it’s this membrane that we call the Reissner’s membrane, and the bottom one is the basilar membrane. So the basilar membrane is an important membrane because the Organ of Corti, where the end organs or the sensitive hair cells, lie on the top of the basilar membrane. And you can see the same in the other illustration over here. So the scala tympani at the bottom, scala vestibuli at the top, and the scala media in between. So the oval window in the vestibule actually opens up into the scala vestibuli. So we’ve go the oval window right there opening up into the scala vestibuli, and the round window opens into the scala tympani over here. So when the stapes moves in and out, it’s actually pushing the fluid, in this case that would be the perilymph, in and out of the scala vestibuli, and it’s a fluid wave that moves around the cochlea to the top and then it comes back all the way down and pushes out the round window. So the scala vestibuli communicates to the middle ear by the oval window and the scala tympani communicates to the middle ear by the round window. So again, the scala vestibuli and the scala tympani are filled by the perilymph, while the scala media that lies in between has this endolymph. Again, in this figure, we’ve got the scala vestibuli, the scala media, and the scala tympani at the bottom over here. At the very top of the cochlea is where actually the scala vestibuli communicates or opens into the scala tympani by this opening called as a helicotrema. However, the scala media that lies in between is kind of sealed from both the scala vestibuli and the scala tympani, so that’s why the chemical composition of the endolymph that lies within here is different from the perilymph that lies within the scala vestibuli and scala tympani. So the scala vestibuli and the scala tympani is filled with perilymph, while the scala media is filled with endolymph. Here’s another illustration of the kind of real cochlea and the scanning image where you are seeing the scala tympani, pardon me, scala vestibuli the scala tympani over here, and between you see the scala media. So it’s this membrane we call as the Reissner’s membrane, and this membrane is what we call as a basilar membrane. Pardon my handwriting, the stylus is pretty bulky. So the basilar membrane separates the scala media from the scala tympani at the bottom, while the Reissner’s membrane separates the scala vestibuli from the scala media. And that’s over here. Reissner’s; basilar membrane over here. At the top of the basilar membrane is where the Organ of Corti lies. The Organ of Corti has these inner hair cells and outer hairs cells where the final auditory transduction happens. Here is an illustration of actually the perilymph and the endolymph. I believe this researcher kind of injected different colored rubber fluid; one through the oval window and one through the round window, I believe. And then he cracked open the skull, and just by the number of turns over here, I’m assuming this is some animal, like a rat or a gerbil because we have more turns than 2 and 3/4 like we see in humans, but it’s a neat way of looking at the 3 different compartments or 3 different scalae of the inner ear.

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